1. Field of the Invention
The present invention generally relates to a way of preventing particles in a plasma from going to a wafer in semiconductor manufacturing processes and, more particularly, to an effective control of the actual wafer potential in a plasma system when it is turned off to keep it slightly negative at all times and thereby reduce and eliminate collection of charged particulates on the wafer.
2. Background Description
In the manufacture of integrated circuits (ICs), semiconductor wafers are subjected to several processes, some of which involve a plasma. Radio frequency (RF) and direct current (DC) glow discharge plasmas, for example, are extensively utilized in the manufacture of ICs. Both electropositive and electronegative gases are used to produce plasmas. Electronegative gases, defined as those having a higher ion density than electron density, such as CF.sub.4, CHF.sub.3, C.sub.12, HBr or O.sub.2, present difficult contamination problems for semiconductor manufacture. Contaminating particles ranging from tenths of microns to microns are produced or grown in the plasmas. The particles normally have a negative charge. Increasing densities of semiconductor circuits make contamination a serious problem to product reliability and may even be a barrier to achieving theoretically possible higher density circuits.
U.S. Pat. No. 5,332,441 to Barnes et al. and assigned to the assignee of this application discloses an apparatus for plasma processing involving the gettering of particles having a high charge to mass ratio away from the wafer. Magnets are used to produce a magnetic field which is transverse to an electric field to draw the negative particles away from the wafer. In addition, a power source is connected to the wafer electrode to maintain a negative charge on the wafer to thereby prevent negative particles from being drawn to the wafer surface when the plasma is turned off. Barnes et al. demonstrated that if the wafer could be kept negative while the plasma was turned off, then the particles in the plasma can be prevented from going to the wafer. However, if a fixed voltage is used, this solution has problems when an insulator is used between the wafer and the wafer electrode. This is the normal case, however, in most systems with a mechanical chuck and in all cases where "electrostatic chucks" are used. These problems are that either a large voltage is used which pulls a large positive charge to the wafer, causing possible breakdown of oxides on the wafer or the insulating layer charges up and allows the wafer to charge to a positive voltage thus attracting the negatively charged particulates in the plasma.